In recent years marked advances have been made in facsimile equipment, printers, etc. In these devices a heat-sensitive recording system using a combination of a heat-sensitive recording paper with a colorless dye such as crystal violet lactone and a phenol compound coated thereon and a thermal head as described in Japanese Patent Publication No. 14039/70 (corresponding to U.S. Pat. No. 3,539,375 and British Patent No. 1,135,540) is widely used.
This heat-sensitive recording system has many advantages. For example, since the recording paper is of the primary coloration type, a developing process is not needed and the recording equipment can be simplified, the recording paper and recording equipment are inexpensive, and since the recording system is of the non-impact type, it does not produce noise. Thus, the heat-sensitive recording system has established its position as a low speed recording system.
The heat-sensitive recording system, however, has a disadvantage in that its recording speed is lower than those of other recording systems, such as electrostatic recording. For this reason it has not yet been employed in high speed recording.
The major reason why high speed recording can not be achieved by the heat-sensitive recording system is that heat conduction between the thermal head and the heat-sensitive recording paper cannot be attained sufficiently and thus a sufficiently high recording density cannot be obtained. The thermal head is a bundle of dot shaped electric resistance heat generators and, upon receipt of a recording signal, generates heat and melts the heat-sensitive color forming layer in contact therewith, thereby allowing the layer to form color. In order to obtain sharp and high density records, it is necessary that dot reproductivity be high. That is, the thermal head and heat-sensitive color forming layer should be brought into as close a contact as possible so that heat conduction is carried out efficiently and on the heat-sensitive color forming layer thoroughly colored dots are formed in full agreement with a high speed recording signal and also with the shape of the dot shaped heat generator. Actually, however, only several percent of the amount of energy generated by the thermal head is conducted to the heat-sensitive color forming layer, and the efficiency of heat conduction is undesirably low.
Several methods have been proposed to increase the smoothness of the heat-sensitive color forming layer so as to bring the thermal head and heat-sensitive color forming layer into as close a contact as possible with each other.
Japanese Patent Publication No. 20142/77, for example, discloses a method of treating the surface of the heat-sensitive color forming layer so that the Beck smoothness is from 200 to 1,000 seconds. Japanese Patent Application (OPI) No. 115255/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application open to public inspection") discloses that if the Beck smoothness is from 200 to 1,000 seconds, the heat-sensitive color forming layer can correspond to heat pulses of only about 5 to 6 milliseconds. Therefore, it is necessary that in the case of high speed recording of 1 millisecond or less the surface of the heat-sensitive color forming layer be smoothed to an extent that the Beck smoothness is 1,100 seconds or more. If, however, the Beck smoothness is increased to more than 1,100 seconds, color fog is formed by the action of pressure. For this reason, in order to prevent the formation of such color fog, the smoothness of an original paper (base paper) to be used is generally increased to an extent that the Beck smoothness is 500 seconds or more. Japanese Patent Application (OPI) No. 156086/80 discloses that the surface roughness (Ra) of the heat-sensitive color forming layer is decreased to 1.2 .mu.m or less, and the glossiness is adjusted to 25% or less.
In accordance with the conventional methods, the smoothness of the heat-sensitive color forming layer has been increased only by application of calendering techniques such as supercalendering, machine calendering, and gloss calendering. This calendering is applied on the base paper alone, or both the base paper and the heat-sensitive paper, or the heat-sensitive paper alone. In the heat-sensitive recording paper, as the smoothness is increased, the recording density increases, but on the other hand, sticking and piling also increase. Therefore, the recording density and the sticking and piling are balanced appropriately by controlling the smoothness to a suitable level. With the conventional heat-sensitive recording papers, even if the smoothness is controlled to any level, they are not suitable for use in high speed recording in both recording density and recording stability.
The term "sticking" as used herein refers to a phenomenon wherein the thermal head sticks to the heat-sensitive color forming layer; therefore, noise is generated when they are separated, and the dot reproductivity is decreased. The term "piling" refers to a phenomenon wherein materials resulting from heat melting of the heat-sensitive color forming layer deposit on the thermal head, thereby decreasing the recording density and dot reproductivity. Both the phenomena interfere with stabilized recording.
Another disadvantage of the application of the calendering onto the heat-sensitive recording paper is that color fog is formed by the action of pressure and the density of the background of the recording paper is increased. Also, the calendering of the base paper is limited in that so-called cockles wrinkles, etc., are formed owing to an unevenness in basis weight.
As described above, smoothing the heat-sensitive color forming layer by application of calendering and increasing the recording density are necessarily limited, and heat-sensitive recording papers fully satisfactory for high speed recording not yet been obtained.